The present invention relates to trackers and particularly to correlation trackers whereby correlation error signals are generated which are indicative of the frame-to-frame correlation of a particular scanned scene.
In general, the correlation tracking concept involves the generation of a cross-correlation function between a stored scene and currently measured scene in which the location of the peak is a measure of the image shift (i.e. the tracking errors). However, if such an approach is followed literally, the operation of systems in which the field of view (FOV) is sampled could be seriously impaired because the correlation between a reference map (preferably generated from an averaging of past sample scenes) and a currently sampled scene will always result in a signal with its peak occurring at discrete values of the resolution elements (i.e. pixels) in the FOV. Clearly, if tracking accuracies several orders of magnitude less than the resolution capability are required, inferior tracking could result using a correlation tracker that relies on correlation peak information.
Conventional methods for obtaining elevation and azimuth tracking error signals (.delta.e and .delta.d respectively) from video data include edge tracking and centroid tracking processes. In the case of a centroid tracker for example, azimuth and elevation weighting functions are essentially stored ramp signals. In order to limit background noise levels, a gating control is often provided to truncate the weighting functions to zero outside the field of view of the gate. Threshold circuits can also reduce the effect of background noise. Attempts are often made to select the gate sizes and threshold levels automatically in order to give an adaptive capability to the tracking system. However, even if such a selection could be done in some optimal fashion, it can be shown that the correlation tracker of the present invention will always yield less noisy tracking error signals. Consequently, the advantages of the correlation tracker of the present invention become most apparent for low signal-to-noise ratios (SNR), i.e., SNR's less than ten, for example.
Systems have also been implemented to perform a correlation tracking function through the use of optics, photographic transparencies, etc. However, such systems have limited flexibility for changing targets. Suggestions have also been promulgated supporting the utilization of digital processing techniques. However, the primary reason that such digital processing has not been developed for a correlation tracker operation has been the inability to efficiently process and use the resultant large amounts of video data. Furthermore, the data rates are often too fast for digital computers to be used in a real time operation.
The video correlation tracker of the present invention circumvents these significant problems in making efficient use of large amounts of data by generating and utilizing near optimal adaptive weighting functions. In theory, it has been demonstrated that the correlation tracker of the present invention has the capability of responding to targets with changing sizes and aspect angles for bandwidths less than .omega..sub.1 .ident. w.sub.1 /T, where T is the frame time for the field of view and w.sub.1 is the weighting parameter for the current scene in a reference map averaging process.